Synapses are specialized sites of cell-cell contact that mediate communication between neurons in the nervous system. Much remains to be discovered about the molecular mechanisms that underlie formation of these critical structures in the mammalian central nervous system. While recent advances have contributed to our understanding of excitatory synapse formation, the processes that mediate inhibitory synapse formation remain poorly defined. In addition, it is hypothesized that aberrant synapse formation and function contributes to neurological disorders such as mental retardation, autism spectrum disorders and epilepsy. To appreciate how synapse dysfunction contributes to these widespread neurological impairments, it is important to first understand how synapses are formed, maintained, and function in the non-pathological state. To this end, we developed a novel, forward genetic RNA interference (RNAi)-based screen in cultured hippocampal neurons that has identified new molecules required for synapse formation. Using this technique, we discovered that RNAi-mediated knockdown of a class 4 Semaphorin, Sema4D, led to a decrease in the density of inhibitory synapses without an apparent effect on excitatory synapse formation. Thus, Sema4D is one of only a few molecules identified thus far that preferentially regulates inhibitory synapse formation. Further, Sema4D appears to be playing a specific role in assembling the postsynaptic specialization at inhibitory synapses. Therefore, understanding the mechanism of action of Sema4D in this process promises to yield key insights into the assembly of inhibitory synapses in the mammalian central nervous system.